The present invention relates generally to an improvement on the apparatuses and methods for magnetically confining molten metal which are disclosed in said antecedent applications. More particularly, this application discloses an improved method and apparatus for preventing the escape of molten metal through the open side of a vertically extending gap between two horizontally separated members and in which the molten metal is located.
The present invention is intended to operate in the same environment as that disclosed in the parent application, e.g., a twin-roll, continuous-casting apparatus. While the apparatus disclosed in the parent application is effective in preventing molten metal from escaping through the open side of a gap between two horizontally separated casting rollers, the improved apparatus of the present invention is designed to accomplish the same task more efficiently.
The twin-roll continuous casting environment in which the present invention is intended to operate typically comprises a pair of horizontally spaced rolls mounted for rotation in opposite rotational senses about respective horizontal axes. The two rolls define a horizontally extending gap therebetween for receiving the molten metal. The gap defined by the rolls tapers in a downward direction. The rolls are cooled, and in turn cool the molten metal as the molten metal descends through the gap.
The gap has horizontally spaced, open opposite ends adjacent the ends of the two rolls. The molten metal is unconfined by the rolls at the open ends of the gap. To prevent molten metal from escaping outwardly through the open ends of the gap, mechanical dams or seals have been employed.
Mechanical dams have drawbacks because the dam is in physical contact with both the rotating rolls and the molten metal. As a result, the dam is subject to wear, leaking, and breakage, and can cause freezing and large thermal gradients in the molten metal. Moreover, contact between the mechanical dam and the solidifying metal can cause irregularities along the edges of metal strip cast in this manner, thereby offsetting the advantages of continuous casting over the conventional method of rolling metal strip from a thicker, solid entity.
The advantages obtained from the continuous casting of metal strip, and the disadvantages arising from the use of mechanical dams or seals are described in more detail in Praeg U.S. Pat. No. 4,936,374 and in Lari et al. U.S. Pat. No. 4,974,661, and the disclosures of each of these patents are incorporated herein by reference.
To overcome the disadvantages inherent in the employment of mechanical dams or seals, efforts have been made to contain the molten metal at the open end of the gap between the rolls by employing an electromagnet having a core encircled by a conductive coil through which an alternating electric current flows and having a pair of magnet poles located adjacent the open end of the gap. The magnet is energized by the flow of alternating current through the coil, and the magnet generates an alternating or time-varying magnetic field extending across the open end of the gap between the poles of the magnet. The magnetic field can be either horizontally disposed or vertically disposed, depending upon the disposition of the poles of the magnet. Examples of magnets which produce a horizontal field are described in the aforementioned Praeg U.S. Pat. No. 4,936,374; and examples of magnets which produce a vertical magnetic field are described in the aforementioned Lari et al. U.S. Pat. No. 4,974,661.
The alternating magnetic field induces eddy currents in the molten metal adjacent the open end of the gap creating a repulsive force which urges the molten metal away from the magnetic field generated by the magnet and thus away from the open end of the gap.
The static pressure force urging the molten metal outwardly through the open end of the gap between the rolls increases with increased depth of the molten metal, and the magnetic pressure exerted by the alternating magnetic field must be sufficient to counter the maximum outward pressure exerted on the molten metal. A more detailed discussion of the considerations described in the preceding sentence and of the various parameters involved in those considerations are contained in the aforementioned Praeg and Lari et al. U.S. patents.
Another expedient for containing molten metal at the open end of a gap between a pair of members is to locate adjacent the open end of the gap a coil through which an alternating current flows. This causes the coil to generate a magnetic field which induces eddy currents in the molten metal adjacent the open end of the gap resulting in a repulsive force similar to that described above in connection with the magnetic field generated by an electromagnet. Embodiments of this type of expedient are described in Olsson U.S. Pat. No. 4,020,890, and the disclosure therein is incorporated herein by reference.
The use of a coil to directly generate the magnetic field adjacent the open end of the gap is more efficient than the use of an electromagnet because when employing an electromagnet, the coil is used to energize the core of a magnet through which magnetic flux must travel to the magnet poles which then generate a magnetic field adjacent the open end of the gap. As a result, there is socalled "core loss" when a coil is employed to energize an electromagnet; but core loss is not a significant factor when the coil is employed to directly generate the magnetic field at the open end of the gap. Even in that case, however, it is important to minimize the energy dissipated by the coil in producing a magnetic field sufficiently strong to confine the molten metal.
A drawback to the latter expedient is that the coil must be placed quite close to the open end of the gap in order to generate a magnetic field which will contain the molten metal there. In the expedient employing an electromagnet, the coil can be relatively remote from the open end of the gap. The closer the coil is to the molten steel, the more severe the thermal conditions to which the coil is subjected. Another drawback to the expedient employing a coil for directly generating the magnetic field at the open end of the gap is that part of the magnetic field is radiated in a direction away from the open end of the gap, thereby decreasing the efficiency of the coil. The problem described in the preceding sentence can also be a problem when employing any electromagnet.
The parent application, Gerber, et al., Ser. No. 07/902,559, discloses a magnetic confining apparatus which employs a single turn coil to directly generate a magnetic field that extends through and is confined substantially to the open side of the gap. In that apparatus, magnetic material encloses all but the front working surface of the front half of the coil, and that magnetic material is used to concentrate current in the working surface of the coil that faces the open side of the gap.
Although the use of such magnetic material is effective in concentrating current in the working surface, it also has several practical limitations.
First, eddy currents induced in the magnetic material by the changing magnetic field produce energy losses and resultant heating of the magnetic material. This effect is minimized by fabricating the magnetic material from thin laminations, but fabrication then becomes more difficult and costly.
Second, the efficiency of the embodiment using magnetic material is further limited by magnetic hysteresis loss in the magnetic material. Magnetic hysteresis loss, a condition which is well-known to those of ordinary skill in the art, refers to energy that is dissipated in the form of heat in magnetic material when a time-varying magnetic field is applied to the magnetic material. Because this energy loss is characteristic of any magnetic material, a molten metal confining apparatus that does not employ magnetic material is desirable.
Each of the above-described energy losses causes heating of the magnetic material. If the current flowing in the coil is strong enough, the heat generated by the above-described energy losses can be severe enough to cause irreversible damage to the magnetic material. Accordingly, there is a limit on the amount of current that can be conducted through the coil, and as a result, there is a corresponding limit on the magnetic confining pressure that can be exerted by the coil. Thus, there is a limit on the amount of molten metal that may be confined by the coil employing magnetic material, in the manner described above, to concentrate current in the working surface. To confine molten metal in amounts exceeding this limit, it is necessary to employ a coil that does not employ magnetic material in such a manner.